Up to now, an optical recording medium, exemplified by an optical disc, has been used as a recording medium for information signals. An optical pickup device is used for writing or reading out information signals on or from an optical recording medium. The optical pickup device includes a semiconductor laser, as a light source for radiating a light beam to be illuminated on the optical recording medium, and an objective lens unit for condensing the light beam radiated from the semiconductor laser for illuminating the light beam to a signal recording surface of the optical recording medium.
In the optical pickup device, the spot diameter of the light beam illuminated on the signal recording surface of the optical recording medium may be reduced to realize high recording density of the information signals recorded on the optical recording medium to enable readout of the information signals recorded to high density.
For reducing the spot diameter of the light beam illuminated on the signal recording surface of the optical recording medium, it is effective to shorten the wavelength of the light beam radiated from the light source and to enlarge the numerical aperture (NA) of the objective lens condensing the light beam.
The present Assignee has proposed an objective lens unit of a larger numerical aperture (NA) in JP Laying-Open Patent Publication H-8-315404 and JP Laying-Open Patent Publication H-10-123410. The objective lens unit disclosed in this Patent publication is composed of a double-lens set made up of two lenses, and has a numerical aperture not less than 0.7.
Up to now, a lens composed of one lens set made up by a single lens, or so-called a “single lens”, has been used extensively as an objective lens unit used in an optical pickup device. The single lens can be prepared by so-called glass mold forming. A lens of high performance can be formed with high reproducibility by fabricating the metal die to high precision and by high precision temperature management during casting. If the lens is to have a larger value of the numerical aperture (NA) of for example 0.7 or larger, a larger refractive power is required of the lens, such that the first surface of the light beam incident side of the lens needs to be a non-spherical surface with a larger curvature. In light of for example mold release properties, it is extremely difficult to form the objective lens unit having a non-spherical surface of a large curvature using a metal die. Moreover, with an objective lens unit having a non-spherical surface of a larger curvature and a larger numerical aperture (NA), the light beam radiated from the light source cannot be condensed accurately on the signal recording surface even on occurrence of perturbations resulting from the slightest tilt relative to the optical axis.
With the objective lens unit comprised of a double-lens set composed of two lenses, as disclosed in the above Publications, the refractive power can be dispersed to two lenses to moderate the curvature of the respective lens surfaces as well as to decrease the non-spherical surface coefficients. Consequently, the objective lens unit can be formed to a desired machining accuracy, using a metal die, so that it becomes possible to suppress deterioration of the optical performance caused by for example the tilt of the lens relative to the optical axis.
With the objective lens unit of a double-lens set composed of two lenses, the respective lenses can be molded with a metal die to prevent its optical characteristics from being deteriorated. However, the respective lenses need to be registered to each other highly accurately, i.e., it is necessary to get the optical axes of the respective lenses of the objective lens unit registered with each other high accurately without producing eccentricity in the respective lenses and to maintain the distance and parallelism between the respective lenses highly accurately.
For producing an objective lens unit of a double-lens set composed of two lenses, there are such a method consisting in causing the laser light to fall on the objective lenses, put together, and in forming an interferometer by the respective lenses to adjust the relative position thereof, and such a method consisting in causing the laser light to be transmitted through the objective lenses put together and in observing the near-field pattern of the laser light to make the adjustment. With these methods, the phenomena observed are not changed independently for respective adjustment parameters, such that adjustment is extremely time-consuming due to many looped procedures required for achieving the final performance.
In assembling, there is such a method which consists in providing a gap between the lens holder 3 and the lens and in adjusting the lens position within the gap range. With this method, an adhesive, such as a UV curable resin, needs to be charged into the gap following the adjustment and cured in situ to secure the lens to the lens holder. The lens secured in position in this manner in the lens holder with an adhesive is likely to undergo misregistration due to environmental changes, such as increasing temperature or humidity.
In order to overcome the problems caused by an adhesive, it has been proposed to set the tilt of the lens and its location along the direction of the optical axis depending on the machining accuracy of the lens holder. That is, a step is formed within the lens holder and the outer rim of the lens is abutted against the step to set the tilt of the lens and its location along the optical axis. If, in this structure, the step is formed high accurately, the lens position can be set similarly accurately.
In the case of, for example, an objective lens unit composed of a double-lens set made up by two lenses, with an effective diameter of 3 mm, it is necessary to maintain parallelism between the two lenses on the order of 0.1 deg. For maintaining this accuracy, it is necessary to maintain the error along the optical axis of the surface of the outer lens rim carried by the step within the lens holder to a value on the order of 1 μm. It is however extremely difficult to have the two lenses mounted within the lens holder as this high degree of accuracy is maintained. Additionally, depending on the mounting environment, fine dust and dirt on the order of 1 μm tend to be intruded into a space between the step within the lens holder and the lens to render it difficult to maintain parallelism between the two lenses.
In the case of an objective lens set, comprised of a double lens set composed of two lenses, the distance between the lens surfaces, comprised of curved surfaces of the lenses, may be kept constant to suppress the generation of spherical aberration ascribable to e.g., errors in lens thicknesses, as disclosed in Japanese Laying-Open Patent Publication H-10-255303.
In the lenses with severe conditions of the relevant standard, the assembling accuracy is optimized by adjustment, as the amount of spherical aberration is detected during assembling, to take up an error caused by variations in accuracy of a metal die used for molding a lens, or in the molding conditions. Although the spherical aberration of the lenses being adjusted may be measured using an interferometer, a complicated apparatus is needed, while the production cost is increased.